Contemporary electrophotographic processes require temperatures of 160.degree.-200.degree. C. to fix toner on a support medium, e.g., a sheet of paper or transparency, creating a developed image. Such high temperatures may reduce or minimize fuser roll life, such as with fuser rolls made of silicone rubbers or fluoroelastomers (e.g., Viton.RTM.), may limit fixing speeds, may necessitate higher power usage during operation, such as in a xerographic copier employing a hot roll fixing mechanism.
Electrophotographic toners are generally prepared by mixing or dispersing a colorant and possibly a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. Known thermoplastic binder resins include polystyrenes, styreneacrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. Carbon black is often used as a colorant and alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and the like are employed as charge enhancing additives.
Although many processes exist for fixing toner to a support medium, hot roll fixing, transferring heat very efficiently, is especially suited for high speed electrophotographic processes. In this method, a support medium carrying a toner image is transported between a heated fuser roll and a pressure roll, the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium, fixing an image.
Toner fixing performance may be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is called the cold offset temperature; the maximum temperature at which the toner does not adhere to the fuser roll is called the hot offset temperature. When the fuser temperature exceeds the hot offset temperature, some of the molten toner adheres to the fuser roll during fixing, is transferred to subsequent substrates (phenomenon known as "offsetting"), and results for example in blurred images. Between the cold and hot offset temperatures of the toner is the minimum fix temperature, which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs. The difference between minimum fix temperature and hot offset temperature is called the fusing latitude.
The hot roll fixing system described above and a number of toners presently used therein exhibit several problems. First, the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption, low fixing speeds, and reduced fuser roll and roll bearing life. Offsetting itself can present a problem.
Toner resin which has a low fix temperature below 200.degree. C. ("low melt toner resin"), preferably below 160.degree. C., exhibits good offset temperature performance. Toners operating at lower temperatures reduce power needs and increase component life. Low melt toners reduce volatilization of release oil such as silicone oil which may occur during high temperature operation and cause problems when the volatilized oil condenses on other areas of the machine. Toners with a wide fusing latitude, providing liberal requirements for oil used as a release agent and improved particle elasticity may minimize copy quality deterioration related to toner offset. Hence, the desirability of low-melt temperature toner resins, particularly for use in hot-roll fixing xerographic processes is apparent.
Investigations have resulted in resins having a lower minimum fix temperature. Such resins may have a lower molecular weight. U.S. Pat. No. 3,590,000 to Palermiti et al. and U.S. Pat. No. 3,681,106 to Burns et al. disclose attempts to use polyester resins as toner binder. These polyester resins exhibit a minimum fix temperature lower than resins made from other materials, such as styrene-acrylic and styrene-methacrylic resins, but may also have an undesirable lower hot offset temperature.
A disadvantage in preparing conventional resins by an organic peroxide initiated reaction mechanism is the relative instability of the cross-linking system used. High temperatures are required to initiate and promote an acceptable reaction rate to achieve a desired degree of cross-linking. These higher temperatures, when producing cross-linked resins on a commercial scale, increase the cost of preparing resins and make them far less profitable commercially.
Skilled artisans, utilizing an organic peroxide aromatic tertiary amine system as a known crosslinking mechanism, have employed the tertiary amines in vinyl polymerization of composite resins, capitalizing on the symbiotic effect tertiary amines exhibit in a binary initiation system. See Brauer et al., "Initiator-Accelerator Systems for Dental Resins," Initiation of Polymerization, F. E. Jr. Ed., American Chemical Society, Washington, DC, p. 359 (1983). However, such applications of the organic peroxide/aromatic tertiary amine (binary initiation system), have been limited to vinyl polymerizations. In one such instance, 2,2'-azobisisobutyronitrile (AIBN) can polymerize vinyl monomers such as: ##STR1##
Studies suggest that free radicals may form on the .alpha.-C atom of the amine as follows: ##STR2##
The UV spectra of the polymer solutions revealed that N,N-di(2-hydroxyethyl)-p-toluidine and N,N-di(2-hydroxypropyl)-p-toluidine moieties are present as end groups of a resulting polymer.
Although peroxide and heterocyclic tertiary amine systems have not received much attention, a drug, such as pilocarpine, containing heterocyclic tertiary amine, i.e., imidazolyl ring can couple with benzoyl peroxide to form a redox initiation system for vinyl polymerization at 40.degree. C. to form a controlled drug delivery device for the drug pilocarpine in film or hydrogel form. See Xin-De Feng, "The Role of Amine in Vinyl Radical Polymerization," Die Makro Molekulare Chemie, 1992, pp. 1-17.
In a more recent study, Uphues et al. (U.S. Pat. No. 4,977,294) proposes a quaternary ammonium phosphate produced by (a) reacting a dicarboxylic acid with an alkoxylated tertiary amine; (b) mixing the aminofunctional polyester obtained, in water, with a mono and/or dialkylphosphoric acid ester; and (c) reacting the mixture of (b) with an alkylene oxide. These quaternary ammonium phosphates are used as anti-static agents for textile materials. The disclosed polymers have low molecular weight structures and are not useful in electrophotographic processes, employing densely cross-linked, high molecular weight polymers.
To date, the inventor believes that functional amines have not been employed in polyester systems, as in the invention. Furthermore, these systems have not previously been proposed for use in toner resins.